(1) Field of the Invention
The present invention generally relates to a subscriber digital transmission system, and more particularly to a subscriber digital transmission system in which information other than information for subscribers can be transmitted using a predetermined time slot in each frame.
(2) Description of the Related Art
In North America, subscribers are scattered over a wide land area, so that it is difficult to provide a plurality of central offices (CO) each of which offices accommodates a plurality of subscribers because of the high cost thereof. Thus, in North America, a subscriber digital transmission system has been proposed in which a central office communicates with subscribers via a remote terminal (RT).
Examples of a structure of the subscriber digital transmission system conventionally proposed are shown in FIGS.1 and 2.
Referring to FIG. 1, a central office (CO) is coupled to a remote terminal 38 (RT) accommodating a plurality of subscriber lines 24 using digital lines. The central office (CO) is provided with a local analog switch 31. Subscriber signals (A) (analog signals) switched by the local analog switch 31 are supplied to channel interfaces (CH INF) 32 in a central office terminal 37. Each of the channel interfaces 32 converts twenty-four analog subscriber signals (A) supplied thereto into twenty-four digital subscriber signals. The digital subscriber signals are supplied to a corresponding one of multiplexer/demultiplexer units (MLDM) 33 along with signaling information. Each of the multiplexer/demultiplexer units 33 multiplexes the twenty-four digital subscriber signals in accordance with a time-division multiplexing method, so that a multiplexed signal having a bit rate of 1.544 Mbps (Megabits per second) is output from each of the multiplexer/demultiplexer units 33. The multiplexing signal (1.544 Mbps) output from each of the multiplexer/demultiplexer unite 33 is supplied to an FP switching unit 34. When a problem occurs in an active transmission line, the FP switching unit 34 switches the signals on the active transmission line to a spare transmission line so as to prevent the communication from being interrupted. A TSI function for changing time slots for subscribers may be added to the FP switching unit 34. The multiplexed signal (1.544 Mbps) output from the FP switching unit 34 is supplied to each of line interfaces 35 (LIN INF). The multiplexed signal is coded by each of the line interfaces 35 in accordance with a predetermined algorithm (e.g. AMI or B8ZS). In each of the line interfaces 35, a unipolar/bipolar conversion process is applied to the multiplexed signal. As a result, a primary rate DS1 or T1 signal (B) is transmitted to the remote terminal 38 via each of the digital lines.
The remote terminal 38 is provided with line interfaces 35, the FP switching unit 34, the multiplexer/demultiplexer units 33 and the channel interfaces in the same manner as the central office terminal 37 in the central Office (CO). the above operations are performed in reverse so that the DS1 or T1 signal is divided into signals for the respective subscribers and the signals are distributed to the subscribers. The signal transmission from the remote terminal 38 to the central office (CO) is performed in the same manner as that described above.
In addition, a transmission operation support system (OS) 40 provided in the central office (CO) supervises via a controller 36 (CONT) alarms and changes of performance state occurring in the central office terminal 37 and the remote terminal 38 and carries out a setting of the operation mode.
In FIG. 2, the central office (CO) is provided with a local digital switch 39 (LDS) substituted for the local analog switch 31 (LAS) shown in FIG. 1. In this subscriber digital transmission system, interfaces in the local digital switch 39 can directly treat the DS1 signals. Operations performed in the system are the same as those in the system shown in FIG. 1.
The primary rate DS1 (or T1) signals standard in North America are transmitted through the digital lines between the central office (CO) and the remote terminal 38. The primary rate DS1 signal has a frame format as shown in FIG. 3 (a). According to the frame format shown in FIG. 3 (a), one frame is formed of a frame bit F.sub.i (i=1, 2, . . . , n) and twenty-four time slots TS assigned to the subscribers. Voice signals, digital data or signaling information for each subscriber is transmitted in a corresponding one of the time slots TS in each frame of the DS1 signal. Each time slot TS contains 8 bits as shown in FIG. 3 (b). Thus, one frame contains 1 bit for the frame bit F.sub.i and 8.times.24=192 bits for the twenty-four time slots. Data of 193 (1+192) bits in each frame is transmitted for 125 microseconds in the digital line. That is, the data is transmitted at the rate of 1.544 Mbps.
A multi-frame includes n frames as shown in FIG. 3 (a). In general, data for one multi-frame is transmitted at once. The number n of frames included in one multi-frame to be transmitted at once depends on the frame format. There are three types of frame formats as shown in FIG. 3 (c). In an SLC-96 format, 72 frames are included (n=72) in one multi-frame, in an SF format, 12 frames are included (n=12) in one multi-frame, and in an ESF format, 24 frames are included (n=24) in one multi-frame. As one multi-frame contains n frames, n frame bits F.sub.i are contained in one multi-frame. In the subscriber digital transmission system, the n frame bits F.sub.i in one multi-frame are used for indicating information. In the case of the multi-frame having the SLC-96 format, the 72 frame bits are used for the synchronizaTION detection, for the detection of signaling information and for data link information for informing the destination of the maintenance and alarm information and the like. In the case of the multi-frame having the SF format, the 12 frame bits are used for the synchronization detection and for the detection of the signaling information. In the case of the multi-frame having the ESF format, the 24 frame bits are used for the synchronization detection, for the detection of the signaling information, for the data link information and for CRC-6 information (a cyclic code) for monitoring the performance state. The ESF format is a recent proposal, and is not very extensively used. On the other hand, the SLC-96 format and the SF format are extensively used.
In a transmission system using the SLC-96 format or the SF format, the bipolar violation (BPV) is used as a scale indicating the state of the transmission system. The unipolar signals handled in the terminals are converted into bipolar signals (information is indicated using positive and negative poles), and the bipolar signals are transmitted through the communication line formed of a T-line and an R-line.
The format of the bipolar signal depends on the coding rule. Two types of the coding rule, an AMI coding rule and a B8ZS coding role, are often used. It is detected using the bipolar violation (BPV) whether or not coding rule errors occur because of the performance of the transmission system (the communication lines and the terminals), and the transmission quality is checked.
In a case where digital services (e.g. DDS and ISDN) are provided in the subscriber digital transmission system, the transmission quality cannot be adequately checked by use of the bipolar violation (BPV) alone, for the following reasons.
The coding rule errors of the bipolar signals can be detected by use of the bipolar violation (BPV). However, bit errors in data cannot be detected by use of the bipolar violation (BPV). That is, the logical errors occurring in data cannot be detected. This is the problem in the case where the digital services (e.g. DDS and ISDN) are performed in the subscriber digital transmission system.
As the logical errors occurring in data cannot be detected by use of the bipolar violation, it is impossible to adequately estimate whether malfunctions are occurring in the subscriber digital transmission system and to switch communications to a spare communication line.
As a result, it is impossible for the transmission operation support system (OS) in the central office (CO) to supervise the transmission quality due to the logical errors in the transmission system.
On the other hand, according to the ESF format, CRC-6 information can be transmitted by use of the frame bits F.sub.i in each multi-frame. The logical errors occurring in the system can be detected by use of the CRC-6 information. However, subscriber digital transmission systems in which the DS1 (T1) signal having the ESF format are transmitted through the communication lines are not widespread. Thus, to detect the logical errors in the transmission system using the CRC-6 information, the switch and transmission equipments in the present system must be replaced with those for the ESF format. In this case, the cost required for replacing the switches and the transmission equipments in all the systems in the country is very high.
Further, in the case of the SF format, the data link information such as alarm information and maintenance information cannot transmitted by the frame bits F.sub.i in each multi-frame. Thus, to transmit the data link information in the subscriber transmission system, the switch and the transmission equipments must be replaced with those for either the SLC-96 format or the ESF format. In this case, the same cost problem arises asfor the above case.